Fix incorrect reference for SHA3_512.

The documentation referenced SHA3_256 instead of SHA3_512.

Related pull request: dotnet/runtime#99860

Co-authored-by: Zavarkó Gábor <zavarkog@gmail.com>

Author: vcsjones

xml/System.Security.Cryptography/SHA3_512.xml

@@ -33,7 +33,7 @@
       </AssemblyInfo>
       <Parameters />
       <Docs>
-        <summary>Initializes a new instance of <see cref="T:System.Security.Cryptography.SHA3_256" />.</summary>
+        <summary>Initializes a new instance of <see cref="T:System.Security.Cryptography.SHA3_512" />.</summary>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>